1. Field of the Invention
The present invention relates to a carbon-based adsorbent comprising active carbon and metallic ions supported on the surface (including internal surfaces) of the active carbon. This adsorbent has adsorbing power similar to that of common active carbon and additionally the ability to remove specific harmful gas molecules and smelling gas molecules from air, thus being especially effective for gas filters.
2. Description of the Prior Art
Having extremely large specific surface area and numerous fine pores in the surface region, active carbon adsorbs gas molecules to great extents and hence is useful as an adsorbent for removing various kinds of gas molecules. However, it cannot be said that active carbon is satisfactory in such functions as that of decomposing or removing ozone contained in exhaust gases and that of adsorbing molecules of ammonia, aldehydes, amines, and the like which are responsible for disagreeable smells.
On the other hand, it is known that transition metal catalysts, e.g. Mn catalysts, are effectively used for decomposing ozone.
Based on such technical background, Japanese Patent Application Laid-Open No. 24340/83, for example, has proposed an adsorbent having active carbon functions combined with said metallic function, which is prepared by coordinating a chelating agent such as ethylenediaminetetraacetic acid (EDTA) with a metal to stabilize it, and causing active carbon to support this chelating compound on the surface.
However, it has been revealed from the present inventors' experimental study that in the invention of the above patent application, most of the used chelating agents have large molecular volumes and hence the whole pore volume of active carbon, when its surface is covered with the resulting chelate compound, is reduced as much as corresponding to the volume of the covering chelate compound, thus lowering the adsorption capacity of the adsorbent. As shown in FIG. 2 of the accompanying drawings, a great number of pores P of about 10-100 .ANG. in size are generally distributed in the surface region of active carbon. The whole volume of these pores, contributing to the adsorption capacity, will be decreased to a great extent by introducing a chelating agent having a large molecular volume.
As examples of the adsorbing power of active carbon fibers (ACF) when a chelate compound is introduced into pores of the fibers, Table 1 shows the specific surface area, benzene-adsorption capacities and ammoniaadsorption capacities of adsorbents prepared by dipping ACF for about 10 minutes in each of L-ascorbic acid-Fe.sup.II chelate solutions of from 1 to 10% varying concentrations, followed by air-drying the fibers, together with the same characteristics of the untreated ACF.
TABLE 1 ______________________________________ Concentration (%) of chelate solution 0 1 2 5 10 ______________________________________ Specific surface 1110 550 240 .apprxeq.0 .apprxeq.0 area (m.sup.2 /g) Benzene adsorption 4.1 1.9 0.8 0.1 .apprxeq.0 capacity (m mol/g) Ammonia adsorption 0.1 2.5 2.1 0.8 0.5 capacity (m mol/g) ______________________________________
As shown in Table 1, the impregnation of ACF with the chelate compound decreases the specific surface area of the ACF. In consequence, the capacity of physical adsorption inherent in ACF is detracted remarkably when ACF is treated with the chelate solutions of such concentrations that the improvement of ACF in ammoniaadsorption capacity is observed.